1. Field of the Invention
The present invention relates to multilayer electronic components and structures for mounting the multilayer electronic components.
2. Description of the Related Art
In recent years, low temperature co-fired ceramic (LTCC) substrates have been in the mainstream of multilayer ceramic substrates. LTCC, which can be fired at low temperatures, namely not more than 1,000° C., allows the use of low-resistance metals such as silver and copper for wiring conductors.
LTCC is provided at reduced firing temperatures and often contains a considerable amount of glass. This material is therefore more brittle than pure ceramics. For example, pure alumina has a flexural strength of about 300 MPa while an LTCC material containing alumina and glass in a volume ratio of 50:50 has a flexural strength of about 200 MPa.
If, therefore, a drop test is performed on a printed circuit board mounting such a multilayer ceramic substrate, a tensile stress occurs at the junctions therebetween to readily cause cracks at the portions of the multilayer ceramic substrate to which the printed circuit board is bonded.
To compensate for such mechanical brittleness, multilayer ceramic substrates have been proposed which have a resin layer that functions as a shock absorber. FIG. 20 is a schematic sectional view of a high-frequency semiconductor device disclosed in Japanese Unexamined Patent Application Publication No. 2003-124435. A multilayer ceramic substrate 102 includes elements such as resistors 108 and chip capacitors. Chip components 103 such as chip resistors and chip capacitors are disposed on the top surface of the multilayer ceramic substrate 102.
Semiconductor elements 101 such as transistors are provided on the bottom surface of the multilayer ceramic substrate 102. A composite resin layer 110 is provided on the bottom surface of the multilayer ceramic substrate 102 such that the semiconductor elements 101 are embedded in the composite resin layer 110. External connection electrodes 104 are provided on the bottom surface of the multilayer ceramic substrate 102 for connection to the electrical circuit of an external printed circuit board 120. The parts such as the semiconductor elements 101 are connected to the external connection electrodes 104 through conductive resins 112 which are via conductors passing through the composite resin layer 110. The external connection electrodes 104 are electrically connected to pad electrodes 121 provided on a main surface of the printed circuit board 120 through bonding members 122.
FIG. 21 illustrates problems in the background art. FIG. 21 is an enlarged sectional view around each external connection electrode 104. A resin layer, for example the composite resin layer 110, is bonded to the multilayer ceramic substrate 102 by heating and pressing a thermosetting resin sheet. The resin layer is bonded to the multilayer ceramic substrate 102 in a semi-solid state (prepreg state) so that the semiconductor elements 101 are embedded in the resin layer.
In the pressing of the resin layer against the multilayer ceramic substrate 102, however, the conductive resins 112, which have been provided inside the composite resin layer 110 in advance, readily protrude because the resin layer is softer than the multilayer ceramic substrate 102. This protrusion leads to the separation between the external connection electrodes 104 and the composite resin layer 110.
In addition, the resin layer containing the via conductors has a small thermal expansion coefficient, namely 11 to 16 ppm/° C., in comparison with that of the via conductors, namely 16 to 20 ppm/° C. When the via conductors and the resin layer rise in temperature, for example, in a reflow process, they differ in the amount of expansion. The via conductors, which expand more than the resin layer, protrude from the bottom surface of the resin layer to partially or completely delaminate the external connection electrodes from the resin layer. That is, the external connection electrodes and the resin layer are completely out of contact and the bonding strength therebetween decreases.
Such decreased bonding strength between the external connection electrodes and the resin layer allows only a slight external force to separate the external connection electrodes from the via conductors. This undesirably results in a defective contact.
If a force is applied in a direction in which the printed circuit board and the multilayer ceramic substrate separate from each other by, for example, dropping the electronic device, a tensile stress acts on the junctions between the external connection electrodes and the resin layer through the solder. In such a case, decreased bonding strength between the external connection electrodes and the resin layer undesirably leads to the delamination of the external connection electrodes and thus a defective contact therebetween.
A second stress tending to delaminate the external connection electrodes is caused by the condensation of solder during its solidification for bonding the multilayer ceramic substrate to a motherboard. In addition, when the external connection electrodes are subjected to Ni—Au plating, a reducing agent in a plating bath for Ni plating produces hydrogen ions which dissolve into Ni. The plating layers therefore solidify with expanded crystal lattices. Afterward, however, the dissolved hydrogen ions are diffused and released to contract the plating layers and decrease the volumes thereof. In the contraction, the plating layers are held by the underlying portions to leave a tensile stress which undesirably delaminates the external connection electrodes and thus causes a defective contact.